1. Field of the Invention
The present invention relates to the preparation of methylenedi(phenylurethane), and, more especially, to the preparation of methylenedi(phenylurethane) by a condensation reaction between an alkyl N-phenylcarbamate and a methylenating agent.
2. Description of the Prior Art
Methylenedi(phenylurethane), designated MDU, is an intermediate useful in the production of methylenedi(phenyl isocyanate), designated MDI. Indeed, MDU can be pyrolyzed to MDI in a manner known per se. MDI is a particularly useful starting material for the manufacture of polyurethane foams and elastomers.
MDI is conventionally produced by phosgenation of the diamine which results from the condensation reaction of aniline with formaldehyde. The commercial product is a mixture of various isomers of MDI and oligomers, designated polymethylenedi(phenyl isocyanate), PMDI, from which the pure MDI is isolated.
For obvious reasons associated with the toxicity of phosgene and the disadvantages associated with the production of hydrochloric acid during the phosgenation step, numerous attempts have been made to prepare MDI by processes which do not require a phosgenation step.
Thus, various processes for the preparation of MDI from alkyl N-phenylcarbamates have been proposed to this art which comprise a first step entailing a condensation reaction of the N-phenylcarbamate with formaldehyde to form a mixture containing diphenylmethane dicarbamate and polymethylenedi(phenyl carbamate), higher homologs of methylenedi(phenyl carbamate) (or MDU), followed by a thermal decomposition step.
One of the disadvantages presented by this type of process is that the proportion of dinuclear MDI, and in particular of the 4,4'-isomer, is insufficient.
Another disadvantage presented by this type of process is that, during the condensation reaction step, significant proportions of compounds such as N-carboalkoxyanilinophenylmethanes, bis(N-carboalkoxyanilino)methanes and N,N'-dicarboalkoxyaminobenzylanilines, as well as their higher condensation derivatives, are formed together with the desired diphenylmethane dicarbamate. These various impurities are problematical in the conversion of the reaction mixture to the desired diisocyanates.
In U.S. Pat. No. 4,146,727 it has been proposed to rearrange impurities of the N-benzyl type of formula (I): ##STR1## in which X, Y, or Z can represent, in particular, a -NHCOOR group and R is an alkyl group having from 1 to 3 carbon atoms, their dimers, trimers, tetramers, etc., into diphenylmethane dicarbamate by contacting them, at a temperature ranging from 50.degree. to 170.degree. C. and preferably from 80.degree. to 130.degree. C., with a catalytically effective amount of a strong protonic acid medium.
Thus, a supplementary step would have to be carried out in order to at least partially suppress such impurities.
In published French Patent Application No. 2,460,972 (corresponding to U.S. Pat. No. 4,319,018) it is proposed to carry out the step involving the condensation reaction of the alkyl N-phenylcarbamate and formaldehyde, or a precursor material generating formaldehyde, in the simultaneous presence of at least one compound selected, in particular, from among bis(N-carboalkoxyanilino)methanes and N,N'-dicarboalkoxyaminobenzylanilines and an aqueous acid solution, the concentration of which is adjusted such that the reaction kinetics are acceptable and secondary reactions are maintained at a minimum level, at a temperature ranging from 10.degree. to 150.degree. C. and preferably from 20.degree. to 120.degree. C.
This process, however, does not permit the removal of all of the impurities under consideration.
Hence, serious need exists in this art for a one-stage process for the preparation of methylenedi(phenylurethane) by a condensation reaction between an alkyl N-phenylcarbamate and a methylenating agent and having an improved selectivity for methylenedi(phenylurethane). Need also exists for a process that permits the proportion of the 4,4'-isomer in the difunctional product to be controlled.